Method for recovering zinc from solution

ABSTRACT

A method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution, the method comprising the steps of: Contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted aqueous ammoniacal ammonium carbonate solution and a zinc-enriched organic solution of a zinc extractant; Separating the zinc-enriched organic solution of a zinc extractant from the zinc-depleted aqueous ammoniacal ammonium carbonate solution; Contacting the zinc-enriched organic solution with an aqueous acidic solution, producing a zinc-enriched aqueous acidic solution and a zinc-depleted organic solution of a zinc extractant; and Recovering zinc from the zinc-enriched aqueous acid solution.

TECHNICAL FIELD

The present invention relates to a method for recovering zinc from an ammoniacal ammonium carbonate solution. More particularly, the present invention relates to a method for recovering zinc from an ammoniacal ammonium carbonate solution by way of solvent extraction.

BACKGROUND ART

The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

Zinc is typically derived from the sulphide mineral sphalerite by roasting the ore to form zinc oxide, leaching the zinc oxide in sulphuric acid, purifying the solution by adding zinc dust and then electrowinning the zinc. Purification of the pregnant leach solution can also be undertaken through the use of a solvent extraction process.

Whilst the processing of zinc ores using an ammoniacal ammonium carbonate leachant is known, the standard method for zinc recovery is precipitation by removal of the ammonia and carbonate by blowing steam through the solution. This precipitate requires further treatment in order to produce zinc cathode. The formation of the intermediate precipitate adds significantly to the complexity and cost of the process.

The broad concept of solvent extraction-based recovery of metals is well known: an aqueous solution containing target metals and impurities is exposed to an organic phase containing an extractant, the extractant allows the metal to be extracted into the organic phase while any impurities largely remain in the aqueous phase, the two phases are separated, the organic phase is then exposed to a clean aqueous phase and the target metal passes into that phase from which it is recovered. However, cost-effective application of the broad principle to a specific aqueous solution of a target metal is rarely straightforward, and the application of solvent extraction principles to aqueous ammoniacal zinc solutions is complicated by several factors. The stability of zinc in aqueous ammoniacal solutions is dependent on pH. At low pH, zinc is soluble. However, at moderate pH zinc precipitates as a hydroxide Zn(OH)₂. At alkaline pH, the zinc hydroxide redissolves as the zincate ion (ZnO₂ ²⁻ or Zn(OH)₄ ²⁻). The addition of ammonia lowers the pH at which the zinc hydroxide redissolves due to the formation of the tetrammine zinc ion Zn(NH₃)₄ ²⁺.

While precipitation presents little impediment to the extraction of zinc from acidic leach solutions, the inventors have discovered that the transfer of protons to the aqueous phase from an acidic organic extractant has a practical effect on the stability of the remaining zinc due to the concomitant reduction in pH. Mitigating this effect solely by the addition of alkali to the aqueous zinc phase has obvious cost implications.

While neutralisation of the organic acid extractant prior to exposure to the aqueous zinc solution, for example with sodium hydroxide, will prevent the pH of the aqueous zinc phase dropping, such an approach has a more complex flow sheet and appreciable cost implications. Even though acid stripping the zinc from the organic extractant regenerates the organic acid extractant, it must be again be neutralised prior to exposure to new aqueous zinc solution.

Alternative approaches have utilized an ammonium salt of the organic extractant which has been demonstrated to reduce the effect on pH of the residual aqueous solution. However, this again necessitates a more complex flowsheet and additional capital and operating cost compared to a process which does not require the pre-treatment of the organic extractant.

Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

SUMMARY OF INVENTION

In accordance with the present invention, there is provided a method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution, the method comprising the steps of:

-   -   i. Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant;     -   ii. Separating the zinc-enriched organic solution of a zinc         extractant from the zinc-depleted aqueous ammoniacal ammonium         carbonate solution;     -   iii. Contacting the zinc-enriched organic solution with an         aqueous acidic solution, producing a zinc-enriched aqueous         acidic solution and a zinc-depleted organic solution of a zinc         extractant; and     -   iv. Recovering zinc from the zinc-enriched aqueous acid         solution.

Throughout this specification, unless the context requires otherwise, the word “extraction” or variations, will be understood to incorporate both chemical and physical transfer of the species undergoing extraction between the aqueous and organic phases.

The inventors of the present invention have discovered that the presence of carbonate in the aqueous ammoniacal ammonium carbonate zinc solution has an effect on the loading of the zinc onto the organic zinc extractant.

As would be understood by a person skilled in the art, ammoniacal solutions are chemically distinct from acid solutions. In an acid leach solution, the level of carbonate will be very low due to the low solubility of CO₂ under the acid conditions necessary for leaching. The addition of carbonates to such a solution therefore results in the evolution of CO₂. In the ammoniacal solutions there is a much higher solubility of bicarbonate/carbonate ions and so CO₂ does not evolve.

In one form of the present invention, the step of:

-   -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant,         more specifically comprises:     -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc and the carbonate is transferred from         the aqueous ammoniacal ammonium carbonate zinc solution,         producing a zinc-depleted carbonate-depleted aqueous ammoniacal         ammonium carbonate solution and a zinc-and carbonate-enriched         organic solution of a zinc extractant;         and step ii) more specifically comprises:     -   Separating the zinc- and carbonate-enriched organic solution of         a zinc extractant and zinc-depleted carbonate-depleted aqueous         solution;         and step iii) more specifically comprises:     -   Contacting the zinc-and carbonate-enriched organic solution with         an acidic solution producing a zinc-enriched aqueous acid         solution, a zinc- and carbonate-depleted organic solution of a         zinc extractant and carbon dioxide;         and step iv) more specifically comprises:     -   Recovering zinc from the zinc-enriched aqueous acid solution.

In a preferred form of the invention, the method further comprises the step of:

-   -   Leaching a zinc-containing ore with an ammoniacal solution to         produce the aqueous ammoniacal ammonium carbonate zinc solution.

Preferably, the step of:

-   -   Leaching a zinc-containing ore with an ammoniacal solution to         produce the aqueous ammoniacal ammonium carbonate zinc solution,         more specifically comprises the step of:     -   Leaching a zinc-containing ore with an ammoniacal solution in         the presence of ammonium carbonate to produce the aqueous         ammoniacal ammonium carbonate zinc solution.

Preferably, the zinc-containing ore contains carbonate minerals. Preferably still, the zinc-containing ore is a zinc carbonate (smithsonite, ZnCO₃, or hydrozincite Zn₅(CO₃)₂(OH)₆) containing ore. With such ores, a portion of the carbonate present in the ore will be dissolved concomitantly with the zinc during leaching in an ammoniacal solution, which may initially contain no carbonate ions, to produce the aqueous ammoniacal ammonium carbonate zinc solution.

As would be recognised by those skilled in the art, it is possible to form an ammonium carbonate solution by the addition of carbon dioxide to a solution of ammonia or by the addition of ammonia to a solution of carbon dioxide or by the addition of ammonia and carbon dioxide to an aqueous solution.

Throughout this specification, unless the context requires otherwise, the term “ore” or variations thereof, will be understood to include, for example, the product of one or more pre-treatment steps, such as a roast or calcination steps, or one or more concentration steps, but is not limited thereto.

In a preferred form of the present invention, where the step of:

-   -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant,         more specifically comprises:     -   Contacting the aqueous zinc ammoniacal ammonium carbonate         solution with an organic solution of a zinc extractant, such         that a portion of the zinc and the carbonate is transferred from         the aqueous ammoniacal ammonium carbonate zinc solution,         producing a zinc-depleted carbonate-depleted aqueous ammoniacal         ammonium carbonate solution and a zinc-and carbonate-enriched         organic solution of a zinc extractant;         the method further comprises the step of:     -   Absorbing the carbon dioxide of step (iii) into an aqueous         solution.

More preferably, the carbon dioxide evolved during step (iii) is absorbed into an aqueous solution containing ammonia. Still preferably, the carbon dioxide evolved during step (iii) is absorbed into the zinc- and carbonate-depleted aqueous ammoniacal ammonium carbonate solution produced during step (i).

In a preferred form the present invention, where the step of:

-   -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant,         more specifically comprises:     -   Contacting the aqueous zinc ammoniacal ammonium carbonate         solution with an organic solution of a zinc extractant, such         that a portion of the zinc and the carbonate is transferred from         the aqueous ammoniacal ammonium carbonate zinc solution,         producing a zinc-depleted carbonate-depleted aqueous ammoniacal         ammonium carbonate solution and a zinc-and carbonate-enriched         organic solution of a zinc extractant;         and the carbon dioxide evolved during step (iii) is absorbed         into an aqueous solution, the aqueous solution of carbon dioxide         is used in the ammoniacal leaching of a zinc-containing ore to         produce the aqueous ammoniacal ammonium carbonate zinc solution         which is the feed to step (i).

In one form of the method of the present invention is performed in a continuous manner, and at least a portion of the organic solution of a zinc extractant of step (i) is provided by the zinc-depleted organic solution of a zinc extractant provided by subsequent method steps.

In one form of the present invention, the method more specifically comprises a method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia, the method comprising the steps of:

-   -   a) Contacting the aqueous ammoniacal ammonium carbonate zinc         solution containing free ammonia with an organic solution of a         zinc extractant, such that a portion of the zinc and ammonia is         transferred from the aqueous ammoniacal ammonium carbonate zinc         solution containing free ammonia, producing a zinc- and         ammonia-depleted aqueous ammoniacal ammonium carbonate solution         and a zinc- and ammonia-enriched organic solution of a zinc         extractant;     -   b) Separating the zinc-enriched ammonia-enriched organic         solution of a zinc extractant and zinc-depleted ammonia-depleted         aqueous solution;     -   c) Contacting the zinc-enriched ammonia-enriched organic         solution with an ammonia scrub solution to reduce ammonia         thereby producing a zinc-rich ammonia-depleted organic solution         of a zinc extractant and an ammonia-enriched solution; and     -   d) Contacting the zinc-enriched ammonia-depleted organic         solution with an acidic solution producing a zinc-rich acid         solution and a zinc-depleted ammonia-depleted organic solution         of a zinc extractant; and     -   e) Recovering zinc from the zinc-rich acid solution.

In one form of the present invention, where the method more specifically comprises a method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia, the ammonia scrub solution preferably has a pH less than 2. More preferably, the ammonia scrub solution has a pH less than 3. Still preferably, the ammonia scrub solution has a pH less than 4. Still preferably, the ammonia scrub solution has a pH less than 5. Still preferably, the ammonia scrub solution has a pH less than 6. Still preferably, the ammonia scrub solution has a pH less than 7.

In one form of the present invention, where the method more specifically comprises a method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia, the ammonia scrub solution preferably has a pH in the range 2-8. Still preferably, the ammonia scrub solution has a pH in the range 3-8. Still preferably, the ammonia scrub solution has a pH in the range 4-8. Still preferably, the ammonia scrub solution has a pH in the range 5-8. Still preferably, the ammonia scrub solution has a pH in the range 6-8. Still preferably, the ammonia scrub solution has a pH in the range 7-8.

Without wishing to be bound by theory, it is believed that the ammonia present in the zinc-enriched ammonia-enriched organic solution is present as weakly-held ammonium NH₄ ⁺ ions. In the presence of protons, the weakly-held ammonium ions are displaced by the more strongly-held protons of the ammonia scrub solution.

In one form of the present invention, where the method more specifically comprises a method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia, the ammonia scrub solution is produced by dilution of the zinc-enriched aqueous acid solution produced in the process. More preferably, the ammonia scrub solution is composed of a dilute solution of a mineral acid, selected from the group: sulphuric acid, hydrochloric acid, nitric acid. In a highly preferred form the ammonia scrub solution is dilute sulphuric acid.

In a preferred form of the present invention, where the method more specifically comprises a method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia, the ammonia scrub solution is carbonic acid, a solution of carbon dioxide in water. More preferably, the concentration of carbonic acid in solution is maintained by constant replenishment of carbon dioxide.

In one form of the present invention, the method more specifically comprises a method for recovering zinc from an aqueous impurity-containing ammoniacal ammonium carbonate zinc solution, the method comprising the steps of:

-   -   a) Contacting the aqueous impurity-containing zinc ammoniacal         ammonium carbonate solution with an organic solution of a zinc         extractant, such that a portion of the zinc and the impurities         from the aqueous impurity-containing ammoniacal ammonium         carbonate zinc solution producing an impurity-depleted         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         an impurity-enriched zinc-enriched organic solution of a zinc         extractant;     -   b) Separating the impurity-enriched zinc-enriched organic         solution of a zinc extractant and impurity-depleted         zinc-depleted aqueous solution;     -   c) Contacting the impurity-enriched zinc-enriched organic         solution with an impurity scrub solution to reduce the         impurities thereby producing an impurity-depleted zinc-enriched         organic solution of a zinc extractant and an impurity-enriched         solution;     -   d) Contacting the impurity-depleted zinc-enriched organic         solution with an acidic solution producing a zinc-enriched         aqueous acid solution and an impurity-depleted zinc-depleted         organic solution of a zinc extractant; and     -   e) Recovering zinc from the zinc-enriched aqueous acid solution

In one form of the present invention, where the method of the present invention more specifically comprises a method for recovering zinc from an aqueous impurity-containing ammoniacal ammonium carbonate zinc solution, the impurity scrub solution preferably has a pH less than 0. More preferably, the impurity scrub solution has a pH less than 1. Still preferably, the impurity scrub solution has a pH less than 2. Still preferably, the impurity scrub solution has a pH less than 3. Still preferably, the impurity scrub solution has a pH less than 4. Still preferably, the impurity scrub solution has a pH less than 5. Still preferably, the impurity scrub solution has a pH less than 6. Still preferably, the impurity scrub solution has a pH less than 7.

Without wishing to be bound by theory, it is believed that the impurities present in the impurity-enriched zinc-enriched organic solution are present in weakly-held forms. In the presence of protons, the weakly-held impurity ions are displaced from the impurity-enriched zinc-enriched organic solution by the more strongly held protons of the impurity scrub solution. As would be recognised by those skilled in the art, different impurities will be held with differing strength and some impurities will require a lower pH impurity scrub solution than other.

In one form of the present invention, where the method of the present invention more specifically comprises a method for recovering zinc from an aqueous impurity-containing ammoniacal ammonium carbonate zinc solution, the impurity scrub solution preferably contains zinc ions, the concentration of zinc in the impurity scrub solution is preferably less than 1 g/L. More preferably, the concentration of zinc in the impurity scrub solution is less than 2 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 3 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 4 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 5 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 7 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 10 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 15 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 20 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 30 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 50 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 60 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 75 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 100 g/L. Still preferably, the concentration of zinc in the impurity scrub solution is less than 150 g/L.

Without wishing to be bound by theory, it is believed that the impurities present in the impurity-enriched zinc-enriched organic solution are present in weakly-held forms. In the presence of zinc ions, the weakly-held impurity ions are displaced from the impurity-enriched zinc-enriched organic solution by the more strongly held zinc ions. As would be recognised by those skilled in the art, different impurities will be held with differing strength and some impurities will require a higher zinc concentration in the impurity scrub solution than others.

As would be recognised by those skilled in the art, the method of the present invention may more specifically comprise both of:

-   -   a method for recovering zinc from an aqueous ammoniacal ammonium         carbonate zinc solution containing free ammonia; and     -   a method for recovering zinc from an aqueous impurity-containing         ammoniacal ammonium carbonate zinc solution,         such that the method of the present invention comprises both the         steps of:     -   Contacting the zinc-enriched ammonia-enriched organic solution         with an ammonia scrub solution to remove ammonia thereby         producing a zinc-enriched ammonia-depleted organic solution of a         zinc extractant and an ammonia-enriched solution;         and     -   Contacting the impurity-enriched zinc-enriched organic solution         with an impurity scrub solution to remove the impurities thereby         producing an impurity-depleted zinc-enriched organic solution of         a zinc extractant and an impurity-enriched solution.

As would be understood by a person skilled in the art, where the method of the present invention may more specifically comprise both of:

-   -   a method for recovering zinc from an aqueous ammoniacal ammonium         carbonate zinc solution containing free ammonia; and     -   a method for recovering zinc from an aqueous impurity-containing         ammoniacal ammonium carbonate zinc solution,         the steps of:     -   Contacting the zinc-enriched ammonia-enriched organic solution         with an ammonia scrub solution to remove ammonia thereby         producing a zinc-enriched ammonia-depleted organic solution of a         zinc extractant and an ammonia-enriched solution;         and     -   Contacting the impurity-enriched zinc-enriched organic solution         with an impurity scrub solution to remove the impurities thereby         producing an impurity-depleted zinc-enriched organic solution of         a zinc extractant and an impurity-enriched solution,         may be performed in either order.

Temperature

In one form of the present invention, the method of the present invention is performed at elevated temperature. Preferably, the step of:

-   -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant         is performed at elevated temperature.

In a preferred form, the method of the present invention is performed at 15-80° C. Still preferably the method of the present invention is performed at 15-70° C. Still preferably, the method of the present invention is performed at 15-60° C. Still preferably, the method of the present invention is performed at 15-50° C. Still preferably, the method of the present invention is performed at 15-40° C. Still preferably, the method of the present invention is performed at 15-30° C. Still preferably, the method of the present invention is performed at 15-25° C. Still preferably, the method of the present invention is performed at ambient temperature. As would be understood by a person skilled in the art, the performance of the method of the present invention at ambient temperature is advantageous as no additional operations costs are incurred by increasing the temperature of the method.

In a preferred form, the method of the present invention is performed at 20-80° C. Still preferably the method of the present invention is performed at 20-70° C. Still preferably the method of the present invention is performed at 20-60° C. Still preferably the method of the present invention is performed at 20-50° C. Still preferably the method of the present invention is performed at 20-40° C. Still preferably the method of the present invention is at 20-30° C.

In a preferred form, the method of the present invention is performed at 25-80° C. Still preferably the method of the present invention is performed at 25-70° C. Still preferably the method of the present invention is performed at 25-60° C. Still preferably the method of the present invention is performed at 25-50° C. Still preferably the method of the present invention is performed at 25-40° C. Still preferably the method of the present invention is performed at 25-30° C.

In a preferred form, the method of the present invention is performed at 30-80° C. Still preferably the method of the present invention is performed at 30-70° C. Still preferably the method of the present invention is performed at 30-60° C. Still preferably the method of the present invention is performed at 30-50° C. Still preferably the method of the present invention is performed at 30-40° C.

Zinc Concentration

Preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 0.1 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 1 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 2 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 3 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 4 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 5 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 7 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 10 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 15 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 20 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 30 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 50 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 60 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 75 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 100 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 150 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 250 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is less than 500 g/L.

In a preferred form of the present invention, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 1 g/L. Preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 2 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 3 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 4 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 5 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 7 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 10 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 15 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 20 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 30 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 50 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 60 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 75 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 100 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 150 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 200 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 250 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 350 g/L. Still preferably, the concentration of zinc in the ammoniacal ammonium carbonate solution is between 0.1 and 500 g/L.

Ammonia Concentration

In one form of the present invention, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 0.1 g/L. Preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 1 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 2 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 3 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 4 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 5 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 7 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 10 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 15 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 20 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 30 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 50 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 60 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 75 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 100 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 150 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 200 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 250 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is less than 300 g/L.

In one form of the present invention, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 1 g/L. Preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 2 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 3 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 4 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 5 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 7 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 10 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 15 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 20 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 30 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 50 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 60 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 75 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 100 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 150 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 200 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 250 g/L. Still preferably, the concentration of free ammonia in the ammoniacal ammonium carbonate solution is between 0.1 and 300 g/L.

Ammonium Carbonate Concentration

In one form of the present invention, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 0.1 g/L. Preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 1 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 2 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 3 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 4 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 5 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 7 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 10 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 15 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 20 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 30 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 50 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 60 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 75 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 100 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 150 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 200 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 300 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is less than 500 g/L.

In one form of the present invention, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 1 g/L. Preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 2 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 3 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 4 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 5 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 7 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 10 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 15 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 20 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 30 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 50 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 60 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 75 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 100 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 150 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 200 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 300 g/L. Still preferably, the concentration of ammonium carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 500 g/L.

Carbonate

In one form of the present invention, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 0.1 g/L. Preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 1 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 2 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 3 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 4 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 5 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 7 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 10 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 15 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 20 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 30 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 50 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 60 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 75 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 100 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 150 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 200 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 250 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 350 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is less than 500 g/L.

In one form of the present invention, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 1 g/L. Preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 2 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 3 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 4 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 5 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 7 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 10 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 15 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 20 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 30 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 50 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 60 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 75 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 100 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 150 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 200 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 250 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 350 g/L. Still preferably, the concentration of carbonate in the ammoniacal ammonium carbonate solution is between 0.5 and 500 g/L.

Aqueous Solution CO₃/Zn Ratio

In one form of the present invention, the step of:

-   -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant         More specially comprises the step of:     -   Controlling the molar ratio of carbonate to zinc in the aqueous         ammoniacal ammonium carbonate zinc solution.

Throughout this specification, unless the context requires otherwise, the word “controlling” or variations, will be understood to include any steps that could be taken to ensure that the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution is within the required range. As would be understood by person skilled in the art, methods of controlling the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution include, but are not limited to:

-   -   adding a soluble carbonate salt, such as those of sodium,         potassium or ammonium; and     -   passing gaseous carbon dioxide through the solution.

In one form of the present invention, the molar ratio of carbonate to zinc in the aqueous solution is less than 0.1. Preferably, the molar ratio of carbonate to zinc in the aqueous solution is less than 0.25. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is less than 0.50. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is less than 0.75. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is less than 0.9. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is less than 1.0. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is less than 1.1. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is less than 1.25. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is less than 1.5. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is less than 2.0.

In one form of the present invention, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 0.1. Preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 0.25. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 0.50. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 0.75. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 0.9. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 1.0. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 1.1. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 1.1. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 1.25. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 1.5. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.05 and 2.0.

In one form of the present invention, the molar ratio of carbonate to zinc in the aqueous solution is between 0.1 and 1.0. Preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.25 and 1.0. More preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.5 and 1.0. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.75 and 1.0. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.9 and 1.0. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.95 and 1.0.

In one form of the present invention, the molar ratio of carbonate to zinc in the aqueous solution is between 0.1 and 1.1. Preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.25 and 1.1. More preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.5 and 1.1. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.75 and 1.1. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.9 and 1.1. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.95 and 1.1.

In one form of the present invention, the molar ratio of carbonate to zinc in the aqueous solution is between 0.1 and 1.5. Preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.25 and 1.5. More preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.5 and 1.5. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.75 and 1.5. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.9 and 1.5. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.95 and 1.5.

In one form of the present invention, the molar ratio of carbonate to zinc in the aqueous solution is between 0.1 and 2.0. Preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.25 and 2.0. More preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.5 and 2.0. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.75 and 2.0. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.9 and 2.0. Still preferably, the molar ratio of carbonate to zinc in the aqueous solution is between 0.95 and 2.0.

The molar ratio of carbonate to zinc in the aqueous solution can be determined using any method available to those skilled in the art. For example, the content of zinc and carbonate in the aqueous phase can be determined by taking an aliquot of the solution and measuring the zinc concentration using, for example, an atomic absorption spectrometer. Carbonate can be measured gravimetrically by precipitation using barium ions or by a pH titration.

Aqueous Solution NH₃/Zn Ratio

Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 0.1. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 0.25. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 0.50. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 0.75. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 0.9. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 1.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 1.1. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 1.25. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 1.5. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 2.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 3.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 5.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 10.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 20.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 50.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is less than 100.0.

Preferably, the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 0.1. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 0.25. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 0.50. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 0.75. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 0.9. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 1.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 1.1. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 1.25. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 1.5. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 2.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 3.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 5.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 10.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 20.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 50.0. Preferably the molar ratio of ammonia to zinc in the aqueous solution is between 0.05 and 100.0.

Zinc Extractant Type

In one form of the invention, the zinc extractant is selected from the group: liquid organophosphorus extractant, liquid oxime extractant, carboxylic acid extractant and combinations thereof.

In a preferred form of the invention, where there is a carboxylic acid extractant, the molecular formula of the carboxylic acid extractant contains 6-14 carbon atoms. In a preferred form of the invention, the molecular formula of the carboxylic acid extractant contains 8-12 carbon atoms.

In a preferred form of the invention the oxime extractant is selected from an aldoxime, a ketoxime or a mixture of both.

In a preferred form of the invention, where there is an organophosphorus extractant, the organophosphorus extractant is a derivative of phosphoric, phosphonic, phosphinic or dithiophosphinic acid.

In a highly preferred form of the invention, where there is an organophosphorus extractant, the organophosphorus extractant is Bis(2-ethylhexyl) hydrogen phosphate (also known as, Bis(2-ethylhexyl) phosphoric acid, Bis(2-ethylhexyl) phosphate, Bis(2-ethylhexyl) hydrophosphoric acid, BEHPA, BEHP, BEHHPA, BEHHP, Di-(2-ethylhexyl) phosphoric acid, D2EHPA, D2EHPA, (C₈H₁₇O)₂PO₂H and has the CAS Number 298-07-7).

The stoichiometry and behaviour of zinc sulphate/organic extractant systems is very well known. For example, in a zinc sulphate—D2EHPA system, two molecules of D2EHPA are required per ion of zinc, i.e. ZnD2EHPA₂, usually written ZnR₂ for brevity. The overall reaction occurring being summarised by (1) where the species in the organic phase are shown italicised and those in plain type in the aqueous phase.

Zn²⁺+2RH=ZnR₂+2H⁺  (1)

The same stoichiometry is universally accepted for the extraction of other divalent metals using D2EHPA (Kolarik and Grimm, Z. Kolarik and R. Grimm, J.Inorg.Nucl.Chem., 1976, 38, 1493-1500; G. M. Ritcey, Solvent Extraction, 2006 and references therein). Without wishing to be bound by theory, it has been discovered by the inventors however that the zinc species extracted from ammoniacal ammonium carbonate is significantly different to that extracted from other aqueous media. Whilst the species extracted from other aqueous media has two molecules of the organic extractant per divalent metal ion, the species extracted from ammoniacal ammonium carbonate solution is understood to extract with only one molecule of the extractant, with the charge being balanced by another monovalent anion in the system, such as hydrogen carbonate (bicarbonate),

Without wishing to be bound by theory, the inventors believe that if the complex RZnHCO₃ is present, then it will be notably shorter than R-Zn-R as there is only a single organic chain on the complex. This is less likely to become entangled than the R-Zn-R complex and therefore not have a substantially higher viscosity than the starting organic solution. This allows the use of a higher concentration of the organic extractant without practical difficulties such as increased separation time.

In one form of the present invention, the molar ratio of ammonia to zinc in the aqueous ammoniacal ammonium carbonate zinc solution is four, or greater. Without wishing to be bound by theory, it is understood by the Applicant that where the molar ratio of ammonia to zinc in the aqueous ammoniacal ammonium carbonate zinc solution is four, or greater, the zinc in the aqueous ammoniacal ammonium carbonate zinc solution will be present as the tetraamine complex ion Zn(NH₃)₄ ²⁺.

In one form of the present invention, the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution is one, or greater. Without wishing to be bound by theory, it is believed that where the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution is one, or greater, the zinc species present in the organic solution following the step of:

-   -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant,         is in the form of ZnRHCO₃, where R is the extractant.

Preferably, at least 10% of the zinc species present in the organic solution is in the form of ZnRHCO₃. More preferably, at least 20% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, at least 30% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, at least 40% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, at least 50% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, at least 60% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, at least 70% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, at least 80% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, at least 90% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, at least 95% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, at least 99% of the zinc species present in the organic solution is in the form of ZnRHCO₃. Still preferably, 100% of the zinc species present in the organic solution is in the form of ZnRHCO₃.

In a highly preferred form the present invention, the molar ratio of ammonia to zinc to carbonate in the aqueous ammoniacal ammonium carbonate zinc solution is about 4:1:1.

Without wishing to be bound by theory, it is believed that the zinc extraction reaction can be summarised as follows, the species in the organic solution are italicized.

Zn(NH₃)₄ ²⁺+HCO₃ ⁻+RH=>ZnRHCO₃+NH₄ ⁺+3NH₃

Extractant Concentration

In one form of the present invention, the concentration of zinc extractant in the organic solution is less than 0.1 vol %. Preferably, the concentration of zinc extractant in the organic solution is less than 1 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 2 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 3 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 4 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 5 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 7 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 10 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 15 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 20 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 30 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 40 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 50 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 60 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 70 vol %. Still preferably, the concentration of zinc extractant in the organic solution is less than 80 vol %.

In one form of the present invention, the concentration of zinc extractant in the organic solution is between 0.5 and 1 vol %. Preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 2 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 3 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 4 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 5 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 7 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 10 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 15 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 20 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 30 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 40 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 50 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 60 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 70 vol %. Still preferably, the concentration of zinc extractant in the organic solution is between 0.5 and 80 vol %.

Phase Modifier

In one form of the invention, the method further comprises the step of:

-   -   adding a phase modifier to the organic solution of a zinc         extractant         prior to the step of:     -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant         in order to inhibit formation of more than two immiscible phases         during the step of:     -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant.

In a preferred form of the invention, where the method further comprises the step of adding a phase modifier to the organic solution of a zinc extractant, the concentration of the phase modifier is less than 0.1 vol %. Preferably, the concentration of the phase modifier is less than 1 vol %. Still, preferably, the concentration of the phase modifier is less than 2.5 vol %. Still preferably, the concentration of the phase modifier is less than 5 vol %. Still preferably, the concentration of the phase modifier is less than 7.5 vol %. In a most preferred form of the invention the concentration of the phase modifier is less than 10 vol %. Still preferably, the concentration of the phase modifier is less than 15 vol %. Still preferably, the concentration of the phase modifier is less than 20 vol %.

In one form of the invention, where the method further comprises the step of adding a phase modifier to the organic solution of a zinc extractant, the concentration of the phase modifier is 0.11-50 vol %. Still preferably, the concentration of the phase modifier is 1-50 vol %. Still preferably, the concentration of the phase modifier is 2.5-50 vol %. Still preferably, the concentration of the phase modifier is 5-50 vol %. Still preferably, the concentration of the phase modifier is 7.5-50 vol %. Still preferably, the concentration of the phase modifier is 10-50 vol %. Still preferably, the concentration of the phase modifier is 15-50 vol %.

In a preferred form of the invention, where the method further comprises the step of adding a phase modifier to the organic solution of a zinc extractant, the phase modifier is an organic alcohol. Preferably, the organic alcohol is selected from the group: 1-decanol, 2-ethyl hexanol, and p-nonyl phenol. In a highly preferred form of the invention the phase modifier is isodecanol (1-decanol, decan-1-ol, CAS 112-30-1, C₁₀H₂₂O).

In a preferred form of the invention, where the method further comprises the step of adding a phase modifier to the organic solution of a zinc extractant, the phase modifier is trioctyl phosphine oxide (TOPO).

In a preferred form of the invention, where the method further comprises the step of adding a phase modifier to the organic solution of a zinc extractant, the phase modifier is tributyl phosphate (TBP, CAS 126-73-8, C₁₂H₂₇O₄P).

Diluent Preferences

In a preferred form of the present invention, the organic solution of a zinc extractant, comprises a zinc extractant and a diluent. Preferably, the diluent is >50% aliphatic. Still preferably, the diluent is >60% aliphatic. Still preferably, the diluent is >70% aliphatic. Still preferably, the diluent is >80% aliphatic. Still, preferably, the diluent is >90% aliphatic Still, preferably, the diluent is >95% aliphatic.

In a preferred form of the invention, the diluent is 50-100% aliphatic. Still preferably, the diluent is 60-100% aliphatic. Still preferably, the diluent is 70-100% aliphatic. Still preferably, the diluent is 80-100% aliphatic. Still preferably, the diluent is 90-100% aliphatic. Still preferably, the diluent is 95-100% aliphatic.

In a highly preferred form of the present invention, the diluent is kerosene.

Organic Solution CO₃/Zn Ratio

In one form of the present invention, the molar ratio of carbonate to zinc in the organic solution is less than 0.1. Preferably, the molar ratio of carbonate to zinc in the organic solution is less than 0.25. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 0.50. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 0.75. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 0.9. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 1.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 1.1. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 1.25. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 1.5. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 2.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 3.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 5.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 10.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is less than 20.0.

In one form of the present invention, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 0.1. Preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 0.25. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 0.50. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 0.75. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 0.9. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 1.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 1.1. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 1.25. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 1.5. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 2.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 3.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 5.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 10.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 20.0.

Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 2.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.25 and 2.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.5 and 2.0. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.5 and 1.5. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.75 and 1.25. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.9 and 1.1. Still preferably, the molar ratio of carbonate to zinc in the organic solution is between 0.95 and 1.05.

The molar ratio of carbonate to zinc in the organic solution can be determined using any method available to those skilled in the art. For example, the content of zinc and carbonate in the organic phase can be determined by taking an aliquot of the organic solution and mixing it thoroughly with a larger volume of 150 g/L sulphuric acid, ensuring that any gaseous carbon dioxide evolved is captured. The phases should then be allowed to separate and an analysis of the aqueous phase for zinc and carbonate ions using standard methods may be performed. To ensure that all of the zinc and carbonate have been stripped, further contacts with fresh 150 g/L sulphuric acid may be required. This method gives the zinc and carbonate concentrations in the organic directly and this data can be used to determine the molar ratio of Zn:CO₃ in the organic.

Organic Solution NH₃/Zn Ratio

Preferably the molar ratio of ammonia to zinc in the organic solution is less than 0.001. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 0.005. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 0.01. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 0.05. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 0.1. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 0.25. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 0.50. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 0.75. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 0.9. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 1.0. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 1.1. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 1.25. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 1.5. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 2.0. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 3.0. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 5.0. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 10.0. Preferably the molar ratio of ammonia to zinc in the organic solution is less than 20.0.

Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 0.1. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 0.25. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 0.50. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 0.75. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 0.9. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 1.0. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 1.1. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 1.25. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 1.5. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 2.0. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 3.0. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 5.0. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 10.0. Preferably the molar ratio of ammonia to zinc in the organic solution is between 0.001 and 20.0.

Organic Solution Zn/R Ratio

In one form of the present invention, the molar ratio of zinc to the zinc extractant in the organic solution is less than 0.1. Preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 0.25. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 0.50. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 0.75. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 0.9. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 1.0. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 1.1. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 1.25. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 1.5. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 2.0. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 3.0. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 5.0. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 10.0. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is less than 20.0.

In one form of the present invention, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 0.1. Preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 0.25. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 0.50. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 0.75. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 0.9. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 1.0. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 1.1. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 1.25. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 1.5. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.05 and 2.0.

In one form of the present invention, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.1 and 1.1. Preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.25 and 1.1. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.5 and 1.1. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.75 and 1.1. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.9 and 1.1. Still preferably, the molar ratio of zinc to the zinc extractant in the organic solution is between 0.95 and 1.05.

In a highly preferred form of the present invention, where the molar ratio of ammonia to zinc to carbonate in the aqueous ammoniacal ammonium carbonate zinc solution is about 4:1:1,

-   -   the concentration of zinc in the aqueous ammoniacal ammonium         carbonate zinc solution is preferably 20-25 g/L;     -   the concentration of ammonia in the aqueous ammoniacal ammonium         carbonate zinc solution is preferably 21-26 g/L;     -   the concentration of carbonate in the aqueous ammoniacal         ammonium carbonate zinc solution is preferably 18-23 g/L;     -   the concentration of the organic solution of a zinc extractant         is 30-40 vol %;     -   the concentration of the phase modifier is 5-10 vol %;     -   the diluent used is greater than 98% aliphatic;     -   the molar ratio of zinc extractant to zinc in the organic         solution is 0.95 to 1.05; and     -   the molar ratio of carbonate to zinc in the organic solution is         0.95 to 1.05,         the zinc species present in the organic solution following the         step of:     -   Contacting the aqueous ammoniacal ammonium carbonate zinc         solution with an organic solution of a zinc extractant, such         that a portion of the zinc is transferred from the aqueous         ammoniacal ammonium carbonate zinc solution, producing a         zinc-depleted aqueous ammoniacal ammonium carbonate solution and         a zinc-enriched organic solution of a zinc extractant,         is in the form of ZnRHCO₃, where R is the extractant.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

FIG. 1 is a flow diagram representing the method of the embodiment, in the context of an aqueous ammoniacal zinc solution produced by the ammoniacal leaching of a zinc carbonate ore;

FIG. 2 is a graphical representation of the loading isotherms for Zn and CO₃ ions from the high carbonate solution of Example 1;

FIG. 3 is a graphical representation of the loading isotherms for Zn and CO₃ ions from the low carbonate solution of Example 2;

FIG. 4 is a graphical representation of the extraction isotherms for 10, 40 and 60 vol % D2EHPA (bottom to top) from solutions where CO3:Zn>=1 in the trials of Example 3;

FIG. 5 is a graphical representation of the Zn/D2EHPA ratios for isotherms of FIG. 4; and

FIG. 6 is a graphical representation of the zinc extraction as a function of equilibrium pH after 1, 2 and 3 contacts in the trials of Example 4.

DESCRIPTION OF EMBODIMENTS

A zinc ore 20 is leached 10 in an ammoniacal ammonium carbonate solution 62 to produce a slurry 27 comprising an aqueous zinc ammoniacal ammonium carbonate solution containing impurities and a zinc-depleted solid, the slurry is subjected to solid-liquid separation 11, to produce a solid waste 41, which is disposed of, and an impurity-containing zinc ammoniacal ammonium carbonate solution 30. The aqueous impurity-containing zinc ammoniacal ammonium carbonate solution 30 is then introduced into a solvent extraction loading stage 12, in which the aqueous impurity-containing zinc ammoniacal ammonium carbonate solution 30 is contacted with an organic solution of a zinc extractant, in the form of D2EHPA, dissolved in an aliphatic 43. The zinc, some impurities and some ammonia are transferred into the organic phase, thereby producing a zinc-depleted impurity-depleted ammonia-depleted aqueous ammoniacal ammonium carbonate solution 60 and a zinc-enriched impurity-enriched ammonia-enriched organic solution of a zinc extractant 33 which are then separated. The zinc-depleted impurity-depleted ammonia-depleted aqueous ammoniacal ammonium carbonate solution 60 is largely recycled back to the leach with a small volume 31 used to absorb the carbon dioxide 59 evolved during the zinc strip 15 forming a carbonic acid solution 32.

The impurity-enriched ammonia-enriched zinc-enriched organic solution of a zinc extractant 22 proceeds to an ammonia scrub extraction 13. In the ammonia scrub stage 13 the impurity-enriched ammonia-enriched zinc-enriched organic solution of a zinc extractant 33 is contacted with an acidic solution of carbonic acid 32, thereby producing an impurity-enriched ammonia-depleted zinc-enriched organic solution 47 and an ammoniacal ammonium carbonate solution 61, which is combined with the raffinate 60 from the SX loading stage 12 to form an ammoniacal ammonium carbonate solution 62 which is recycled back to the leach stage 10. The impurity-enriched ammonia-depleted zinc-enriched organic solution 47 is then contacted with an acidic aqueous solution containing zinc 56 in an impurity scrub stage 14 forming an acidic aqueous solution containing zinc and impurities 58, which can be disposed of and an impurity-depleted ammonia-depleted zinc-enriched organic solution of a zinc extractant 49. The impurity-depleted ammonia-depleted zinc-enriched organic solution of a zinc extractant 49 proceeds to a zinc strip stage 15 where it is contacted with a strongly acidic solution 51 producing a zinc-enriched acid aqueous phase 52, an impurity-depleted ammonia-depleted zinc-depleted organic solution of a zinc extractant 43 and carbon dioxide 59. The zinc-enriched acid aqueous phase 52 proceeds to zinc electrowinning 16 where zinc cathodes 53 are produced. The impurity-depleted ammonia-depleted zinc-depleted organic solution of a zinc extractant 43 is recycled back to the SX loading stage 12. The carbon dioxide 59 is absorbed into an aqueous solution 31 and the resultant solution 32 used in the ammonia scrub 13.

EXAMPLE 1

A zinc silicate ore was leached with an ammoniacal ammonium carbonate solution consisting of ˜25 g/L free ammonia and ˜8 g/L of ammonium carbonate. After 24 h of leaching, the solution was separated by filtration, the zinc tenor of this solution was 4.67 g/L. An organic phase comprising 100 mL of D2EHPA diluted to 1.00 L with aliphatic kerosene (Recosol V80), giving a solution containing 0.303 M D2EHPA, was made up.

A small volume of the aqueous zinc solution was shaken with an aliquot of the organic D2EHPA solution for several minutes. The phases were allowed to separate and the aqueous phase drained off and analysed for zinc and carbonate ions using standard methods. The masses of zinc and carbonate ions transferred into the organic phase was calculated as the difference between the feed solution (4.67 g/L) and the solution after contact with the organic phase.

A fresh aliquot of aqueous zinc solution was then contacted with the same aliquot of organic D2EHPA solution and the cycle repeated until there was little further extraction of zinc from the aqueous solution.

The data points and best-fit Langmuir isotherms for zinc and carbonate ions resulting from this work are shown in FIG. 2. The carbonate data and isotherms are triangles and dashed line respectively.

The Langmuir isotherm indicates that the maximum zinc loading in the organic solution is 19.04 g/L, (0.291 M) and the maximum carbonate loading is 17.53 g/L (0.292 M). Thus the molar ratio of CO₃:Zn in the organic phase is 0.292/0.291=1.003. Since the D2EHPA concentration was 0.303 M the molar ratio of R:Zn:CO₃=0.303:0.291:0.292=1.04:1:1 or ˜1:1:1.

EXAMPLE 2

The leaching run in Example 1 was repeated using ˜1 g/L of ammonium carbonate, this being the only difference between the two runs. The solution tenor for zinc of 4.60 g/L is very close to that from Example 1, evidently, the level of carbonate ions in the leach solution has little effect on the extent of leaching from this ore. The extraction process and analyses were also repeated to give the isotherms shown in FIG. 3.

The isotherms for zinc and carbonate ions are shown in FIG. 2. The maximum loadings of zinc and carbonate ions were 10.203 g/L (0.156 M) and 0.491 g/L (0.00818 M) respectively. This leads to a molar ratio of Zn:CO₃ of 0.156/0.00818=19.1. The D2EHPA concentration of 0.303 M leads to the molar ratio of R:Zn=0.303:0.156=1.94:1.

Simple comparison with Example 1 shows that reducing the molar ratio of carbonate ions to zinc ions in the aqueous feed solution from 1.18 to 0.15 reduced the maximum extraction of zinc from 19.04 g/L to 10.20 g/L, a very substantial reduction. Clearly, the presence of carbonate ions during the extraction have a very significant effect on zinc extraction.

Without wishing to be bound by theory, the low level of ammonium carbonate in the aqueous solution (1.0 g/L, 0.010 M) is clearly insufficient to enable the transfer of more than ˜6.4% (0.010/0.156 *100) of the zinc into the organic phase as the proposed ZnRHCO₃ complex. The remaining ˜93.6% of the zinc will be extracted as the well-known ZnR₂ complex. On this basis, the average molar ratio of D2EHPA to Zn can be calculated to be (6.4*1+93.6*2)/100=193.6/100=1.936. This is extremely close to the experimental value of 1.94 derived from the isotherm.

EXAMPLE 3

A zinc bearing ammoniacal ammonium carbonate solution was made by leaching a dolomite ((Ca,Mg)CO₃) hosted smithsonite (ZnCO₃) ore in an ammoniacal solution for 24 h. After this time the solution was filtered to eliminate solids. The resultant solution contained 19 g/L of zinc, 30 g/L of total ammonia (i.e. ammonia+ammonium) and ˜29 g of carbonate ions. This gives a carbonate to zinc molar ratio of 1.04.

Two organic solutions were prepared with either 40 or 60 vol % D2EHPA in addition to 5 vol % TBP, the balance being kerosene. The aqueous and organic solutions were shaken together at different volume ratios, allowed to settle and the aqueous solution analysed for zinc. The zinc concentration in the organic solution was calculated by difference.

FIG. 4 show the extraction isotherms for 40 and 60 vol % D2EHPA along with the isotherm for 10 vol % in high carbonate solution from Example 1. The experimental data are shown as points, the lines show a Langmuir isotherm calculated from the data.

The composition of the species present in the organic solution at saturation can be determined from the maxima of the Langmuir isotherms. The molar ratio of zinc to D2EHPA in the organic phase can be calculated. 40 vol % D2EHPA is ˜1.21 M, 83 g/L Zn is 1.27 M, thus the molar ratio Zn/D2EHPA is 1.05. Similarly, in the 60 vol % D2EHPA run, the ratio is 1.02. Accordingly, it appears that the zinc species extracted from ammoniacal ammonium carbonate can be summarised as ZnR, which is significantly different to the ZnR₂ species extracted from other aqueous media.

As would be recognised by those skilled in the art, the Langmuir isotherm can also be used to determine the Zn/D2EHPA ratio for any level of loading of the organic. This data is shown in FIG. 5 for all three isotherms from FIG. 3.

The Zn/D2EHPA ratio for the individual data points shown in FIG. 4 ranges from 0.09 to 1.05. In the former case, only one in every eleven D2EHPA molecules present in the organic is attached to a zinc ion, the remaining ten are available to complex with other zinc ions as they are extracted in subsequent stages. This is made clear by the increasing Zn/D2EHPA ratio as the quantity of zinc in the organic phase increases with increasing numbers of contacts with the leach solution.

Those skilled in the art will recognise that the most economic and efficient utilisation of the D2EHPA will be where the maximum number of D2EHPA molecules are associated with zinc ions, i.e. at the upper end of the D2EHPA saturation where the zinc concentration in organic as a percentage of the maximum possible zinc concentration is close to 100% and the Zn/D2EHPA ratio is highest. It would be possible, but economically poor to operate at much less than 50% saturation where the Zn/D2EHPA ratio is <0.5.

Without wishing to be bound by theory, it is clear that when the D2EHPA is not at maximum saturation there will be D2EHPA molecules in the organic phase which are not associated with the zinc ions. Under such circumstances the Zn/D2EHPA ratio must always be <1 and at the lowest saturations the amount of free D2EHPA present will be highest and the Zn/D2EHPA ratio lowest.

Without wishing to be bound by theory, it is believed that the species extracted from ammoniacal ammonium carbonate solutions also contains a hydrogen carbonate. The reason is twofold. Firstly, the valence of zinc is unchanged during extraction and a univalent anion is required to balance the charge in both aqueous and organic solutions. In the aqueous solution the only anion present at any useful concentration is carbonate. Secondly, between pH 6.2 and 10.2 the carbonate is predominantly present as hydrogen carbonate (bicarbonate), HCO₃ ⁻. Thus it would seem that the zinc-bearing species present in the organic solution after extraction from ammoniacal ammonium carbonate is RZnHCO₃.

It is well known by those skilled in the art that using the concentration of D2EHPA used to extract zinc from aqueous acid sulphate solutions has an upper practical limit of around 40 vol %. Above this level, the viscosity of the organic solution is too high for use in an industrial operation. Even at this concentration, the solvent extraction circuit is typically operated at elevated temperature to reduce viscosity problems. The increased viscosity has been attributed to formation of polymers of the ZnR₂ complexes (Z. Kolarik and R. Grimm, J.Inorg.Nucl.Chem., 1976, 38, 1721-1727). The D2EHPA molecules attach to opposite sides of a square plane surrounding the zinc effectively forming a much longer chain, which a structural formula of R-Zn-R. The longer chains more readily become entangled raising the viscosity to the point where it is too high for industrial use.

A recent publication (Phys. Probl. Miner. Proc. 50(1), 2014, 311-325) examines zinc recovery from a 60 g/L Zn solution using 36 vol % D2EHPA. They note that using 36 vol % D2EHPA was unsatisfactory to use “Due to high viscosity” and that as a consequence, “the concentration in further experiments has been limited to 18% (vol.)”.

The present system showed no apparent viscosity problems, even when using 60 vol % D2EHPA as the extractant. If the putative complex RZnHCO₃ is present, then it will be notably shorter than R-Zn-R as there is only a single organic chain on the complex. This is less likely to become polymerised than the R-Zn-R complex and therefore not have a substantially higher viscosity than the starting organic solution.

EXAMPLE 4

A large volume of zinc bearing aqueous solution was prepared by leaching 24 kg of 35% Zn ore in 300 L of ˜35 g/L free ammonia solution to give a zinc tenor of ˜16.7 g/L. The ore also contained leachable lead and cadmium. Fresh aqueous at a starting pH of 10.91 was contacted at a 1:1 volume ratio with fresh organic (40 vol % D2EHPA+10 vol % TBP in kerosene), the organic removed, the aqueous sampled and replaced with fresh organic. This was repeated three times. FIG. 4 shows the zinc extraction as a function of equilibrium pH after 1, 2 and 3 contacts.

Despite the high starting free ammonia in the PLS, it is clear that three contacts were sufficient to reduce the pH to a level where the metals extraction was high.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, formulations and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features. 

1. A method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution, the method comprising the steps of: i. Contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted aqueous ammoniacal ammonium carbonate solution and a zinc- enriched organic solution of a zinc extractant; ii. Separating the zinc-enriched organic solution of a zinc extractant from the zinc-depleted aqueous ammoniacal ammonium carbonate solution; iii. Contacting the zinc-enriched organic solution with an aqueous acidic solution, producing a zinc-enriched aqueous acidic solution and a zinc-depleted organic solution of a zinc extractant; and iv. Recovering zinc from the zinc-enriched aqueous acid solution.
 2. The method according to claim 1, wherein the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution is controlled to between 0.05 and 2.0.
 3. The method according to claim 1, wherein a portion of the carbonate is transferred from the aqueous ammoniacal ammonium carbonate zinc solution to organic solution of a zinc extractant, such that the method more specifically comprises the steps of: i. Contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc and the carbonate is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted carbonate-depleted aqueous ammoniacal ammonium carbonate solution and a zinc-and carbonate-enriched organic solution of a zinc extractant; ii. Separating the zinc- and carbonate-enriched organic solution of a zinc extractant and zinc-depleted carbonate-depleted aqueous solution; iii. Contacting the zinc-and carbonate-enriched organic solution with an acidic solution producing a zinc-enriched aqueous acid solution, a zinc- and carbonate-depleted organic solution of a zinc extractant and carbon dioxide; and iv. Recovering zinc from the zinc-enriched aqueous acid solution.
 4. The method according to claim 1, wherein the method further comprises the step of: Leaching a zinc-containing carbonate-containing ore with an ammoniacal solution to produce the aqueous ammoniacal ammonium carbonate zinc solution.
 5. The method according to claim 4, wherein the zinc-containing ore is leached with an ammoniacal solution in the presence of ammonium carbonate to produce the aqueous ammoniacal ammonium carbonate zinc solution.
 6. The method according to claim 3, wherein the carbon dioxide produced in the step of: Contacting the zinc-and carbonate-enriched organic solution with an acidic solution producing a zinc-enriched aqueous acid solution, a zinc- and carbonate-depleted organic solution of a zinc extractant and carbon dioxide is absorbed into an aqueous solution to produce an aqueous solution of carbon dioxide.
 7. The method according to claim 6, wherein the carbon dioxide is absorbed into an aqueous solution containing ammonia.
 8. The method according to claim 6 or 7, wherein the carbon dioxide is absorbed into the zinc- and carbonate-depleted aqueous ammoniacal ammonium carbonate solution produced during step (i).
 9. The method according to claim 6, wherein the aqueous solution of carbon dioxide is used in the ammoniacal leaching of a zinc-containing ore to produce the aqueous ammoniacal ammonium carbonate zinc solution which is the feed to step (i).
 10. The method according to claim 1, wherein the aqueous ammoniacal ammonium carbonate zinc solution contains free ammonia, such that the method more specifically comprises a method for recovering zinc from an aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia, the method comprising the steps of: i. Contacting the aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia with an organic solution of a zinc extractant, such that a portion of the zinc and ammonia is transferred from the aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia, producing a zinc- and ammonia-depleted aqueous ammoniacal ammonium carbonate solution and a zinc- and ammonia-enriched organic solution of a zinc extractant; ii. Separating the zinc-enriched ammonia-enriched organic solution of a zinc extractant and zinc-depleted ammonia-depleted aqueous solution; iii. Contacting the zinc-enriched ammonia-enriched organic solution with an ammonia scrub solution to reduce ammonia thereby producing a zinc-rich ammonia-depleted organic Contacting the zinc-enriched ammonia-depleted organic solution with an acidic solution producing a zinc-rich acid solution and a zinc-depleted ammonia-depleted organic solution of a zinc extractant; and iv. Recovering zinc from the zinc-rich acid solution.
 11. The method according to claim 10, wherein the ammonia scrub solution preferably has a pH in the range 2 to
 7. 12. The method according to claim 10, wherein the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia is controlled to between 0.05 and 2.0.
 13. The method according to claim 1, wherein the aqueous ammoniacal ammonium carbonate zinc solution contains impurities, such that the method more specifically comprises a method for recovering zinc from an aqueous impurity-containing ammoniacal ammonium carbonate zinc solution, the method comprising the steps of: i. Contacting the aqueous impurity-containing zinc ammoniacal ammonium carbonate solution with an organic solution of a zinc extractant, such that a portion of the zinc and the impurities from the aqueous impurity-containing ammoniacal ammonium carbonate zinc solution producing an impurity-depleted zinc-depleted aqueous ammoniacal ammonium carbonate solution and an impurity-enriched zinc-enriched organic solution of a zinc extractant; ii. Separating the impurity-enriched zinc-enriched organic solution of a zinc extractant and impurity-depleted zinc-depleted aqueous solution; iii. Contacting the impurity-enriched zinc-enriched organic solution with an impurity scrub solution to reduce the impurities thereby producing an impurity-depleted zinc-enriched organic solution of a zinc extractant and an impurity-enriched solution; iv. Contacting the impurity-depleted zinc-enriched organic solution with an acidic solution producing a zinc-enriched aqueous acid solution and an impurity-depleted zinc-depleted organic solution of a zinc extractant; and v. Recovering zinc from the zinc-enriched aqueous acid solution
 14. The method according to claim 11, wherein the impurity scrub solution preferably has a pH in the range 0 to
 5. 15. The method according to claim 11, wherein the molar ratio of carbonate to zinc in the aqueous ammoniacal ammonium carbonate zinc solution containing free ammonia is controlled to between 0.05 and 2.0
 16. The method according to claim 10, wherein the aqueous ammoniacal ammonium carbonate zinc solution contains both free ammonia and impurities, where the method more specifically comprises both the steps of: Contacting the zinc-enriched ammonia-enriched organic solution with a ammonia scrub solution to remove ammonia thereby producing a zinc-enriched ammonia-depleted organic solution of a zinc extractant and an ammonia-enriched solution; and contacting the impurity-enriched zinc-enriched organic solution with an impurity scrub solution to remove the impurities thereby producing an impurity-depleted zinc-enriched organic solution of a zinc extractant and an impurity-enriched solution.
 17. The method according to claim 1, wherein the molar ratio of ammonia to zinc in the aqueous solution is between 3 and
 20. 18. The method according to claim 1, wherein the zinc extractant is selected from the group: liquid organophosphorus extractant, liquid oxime extractant, carboxylic acid extractant and combinations thereof.
 19. The method according to claim 1, wherein the zinc extractant is Bis(2-ethylhexyl) hydrogen phosphate.
 20. The method according to claim 1, wherein the method further comprises the step of: adding a phase modifier to the organic solution of a zinc extractant prior to the step of: contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted aqueous ammoniacal ammonium carbonate solution and a zinc-enriched organic solution of a zinc extractant
 21. The method according to claim 1, wherein the organic solution of a zinc extractant, comprises a zinc extractant and a diluent.
 22. The method according to claim 1, wherein the molar ratio of carbonate to zinc in the organic solution is between 0.05 and 2.0.
 23. The method according to claim 1, wherein the molar ratio of ammonia to zinc to carbonate in the aqueous ammoniacal ammonium carbonate zinc solution is about 4:1:1.
 24. The method according to claim 1; wherein at least 10% of the zinc species present in the organic solution following the step of: Contacting the aqueous ammoniacal ammonium carbonate zinc solution with an organic solution of a zinc extractant, such that a portion of the zinc is transferred from the aqueous ammoniacal ammonium carbonate zinc solution, producing a zinc-depleted aqueous ammoniacal ammonium carbonate solution and a zinc-enriched organic solution of a zinc extractant, is in the form of ZnRHCC>3, where R is the extractant. 